1. Field of the Invention
The present invention relates to an input device, which can detect a position close to an operator on the basis of a variation in an electrostatic field between electrodes. In particular, this invention relates to an input device adapted to update a reference value indicative of an amount of variation in the electrostatic field.
2. Description of the Related Art
Japanese reference JP-A-08-137607 described below, includes a coordinate input device of an electrostatic capacitance type having a plurality of X electrodes and Y electrodes arranged in a matrix.
In this coordinate input device, the plurality of X electrodes are arranged on a surface of a glass substrate, the plurality of Y electrodes are arranged on the other surface of the glass substrate, X electrodes and Y electrodes are disposed in the matrix through the glass substrate, and a predetermined electrostatic capacitance is formed between each of the X electrode and the Y electrode.
Through a control unit, the X electrodes are connected to a transmission circuit and a Y electrode enters an ON state such at a predetermined potential is applied on each of the X electrodes and Y electrodes. At this state, when a finger of the user touches the coordinate input device and the device is pushed, the electrostatic field between the X electrodes and Y electrodes is varied, and a voltage change due to the variation in the electrostatic field is output from each of the Y electrodes. The voltage changes output from each of the Y electrodes are read by the control unit via an A/D converting means. Voltage detection means provided in the control unit specifies a portion where the electrostatic field between X electrode and Y electrode are adjacent to each other on the basis of data outputted from the A/D converting means, thereby detecting positional information corresponding to the portion touched by the user's finger.
The input device of the electrostatic capacitance type detects the variation in the electrostatic field when the device is touched with the finger, which is similar to a conductive material. However, in order to detect the touching of the finger with high sensitivity, it is necessary to detect an extremely small variation in the electrostatic capacitance between electrodes. Because the electrostatic capacitance between electrodes subtly varies depending on changes in its environment or variations in floating capacitance, a erroneous operation may occur. In an erroneous operation, a position different from a true coordinate position of the user's finger is detected.
Therefore, the input device of the electrostatic capacitance type is generally provided with a correction function. In the correction function, for example, when it is determined that the finger approached a certain portion of the input device, and then it is determined that the finger moves away from that position, a comparison reference value is updated.
However, the correction means for updating the reference value when the finger moves away cannot correct for an erroneous operation, such as when the user's finger does not get sufficiently close to the portion of the input but the detection output indicates that the finger is close. Such the erroneous operation may be caused by an expansion and contraction of the substrate due to drastic temperature changes, or due to a deformation of the substrate due to the temperature change, or due to an external force. When the substrate is expanded or deformed, a distance between electrodes formed on the substrate is locally changes so that the electrostatic capacitance between electrodes is physically changed. In this state, when a potential is applied to the electrode, a local change in capacitance is detected, and a variation in detected values is continued for a predetermined time. At this time, an electronic circuit erroneously recognizes that a predetermined portion of the input device is operated by the finger, and thus an erroneous detection is continuously outputted.
When the input device is employed in an electronic apparatus that experiences severe temperature changes, or a linear expansion coefficient of the substrate is large, such erroneous operation may easily occur. For example, when the substrate is made of a resin sheet and is employed in a portable apparatus, and the portable apparatus is moved from an interior environment equipped with air conditioning, to a high temperature external environment, the erroneous operation may occur.